1. Field of the Invention
The present invention relates to a method of regulating optical axes of two headlights for an automotive vehicle, and more specifically to a method of regulating optical axes of low beam of the headlights.
2. Description of the Prior Art
In an automotive vehicle assembly line, there is required a process of regulating optical axes of two headlights so that the light intensity boundary line (referred to as cut line) of the low beam thereof lies within a predetermined standard range, by projecting the low beam of the headlights on a frontward screen after the headlight has been assembled.
FIG. 1 shows an example of a light intensity distribution pattern of the low beam of headlights projected on a frontward screen, in which L denotes equi-illumination intensity lines; M denotes the maximum illumination intensity point; and K denotes a standard range of the light intensity distribution pattern, by way of example. Therefore, the optical axes of the headlights are regulated so that the cut line C (composed of a horizontal cut line C.sub.1 and an oblique cut line C.sub.2) indicative of boundary between a bright area a and a dark area b lies within the standard range K, as depicted in FIG. 1.
So far, since the above-mentioned optical axis regulating work for headlights has-been conducted by visual inspection, there exists a problem in that the regulation precision cannot be improved beyond a limit and further the worker's eye tends to become fatigued. To overcome this problem, recently, a method of regulating the optical axes has been proposed such that the headlight low beam is projected on a screen; the projected low beam is image detected by video cameras; detected video signals are image processed and displayed on a monitor TV to display a light distribution pattern including the light intensity boundary line (the cut line); and the optical axes are regulated so that the displayed cut line lies within a standard range on the monitor TV.
In more detail, in the case of a method disclosed in Japanese Published Unexamined (Kokai) Patent Appli. No. 63-113339, for instance, a light intensity distribution pattern of the low beam of headlights is formed on a screen disposed in front of the vehicle; a gravity center of equiillumination intensity closed-curves of a low beam having brightness beyond a predetermined value is calculated by an image processing apparatus; the image of the distribution pattern is differentiated in a vertical direction along a line passing through the gravity center; a light intensity boundary point in the vertical direction is determined on the basis of the differentiated values; the cut line is detected by binarization of the image signals, with the brightness of the light intensity boundary point as threshold; a relative distance between the gravity center and a point of intersection between the horizontal and oblique cut lines is previously obtained; a phantom cut line is determined on the basis of the gravity center and the relative distance; and the phantom line is checked as to whether lying within a standard range.
In the prior-art method as described above, however, since the cut line (light intensity boundary line) is detected by binarization of the -image signals, with the brightness of only the single light intensity boundary point as threshold, there exists a problem in that the light intensity boundary point is not determined accurately due to noise and therefore the cut line is not detected correctly, so that error is inevitably produced in the optical axis regulation. In addition, although the cut line is detected on the basis of the actual image signals at the first regulating work, since the cut line is determined on the basis of a phantom line obtained by the gravity center and the relative distance at the second and after regulating work, when sufficient image sensing conditions are not satisfied at the first work, there exists a problem in that the cut line (the light intensity boundary line) will not be detected correctly.